This invention relates to a double-beam spectrophotometer.
In a typical double-beam spectrophotometer with a single detector, the monochromatic light emerging from a monochromator is caused by a rotating sector mirror to alternately advance along two separate optical paths, the beams on which are referred to as the sample and reference beams, in which a sample and a sample cell are disposed respectively. The two optical paths are again combined into a single path, along which the sample and reference beams are caused by a beam combiner to alternately advance to a single detector such as a photomultiplier tube.
The above arrangement is suitable for measurement of those liquid samples which do not scatter light. However, it is not suitable for measurement of a sample having light scattering property or a sample such as an optical instrument itself just assembled or a sample which must be kept at a particular temperature different from room temperature.
In particular, if a light scattering sample is to be measured, it must be disposed as close to the detector as possible. Between the sample and the detector, however, there must be a considerable optical distance or a structural space enough to accommodate several optical elements such as a sector mirror or other mirrors, so that most of the light scattered by the sample is lost before it reaches the detector.
Given samples vary in size and shape, and for measurement of large-sized samples the instrument must have as large a space as possible to accommodate a large-sized sample. The optical system of a typical double-beam spectrophotometer, however, provides only a limited space between two sector mirrors for a sample to be put in, so that there is a limitation to the size and shape of the sample that can be set in the instrument for measurement.
The same difficulty is encountered when a sample cell is provided with an attachment such as a constant temperature bath or a cell positioner.